1. Field of the invention
The present invention relates to an integrated optics polarizing device, based on a structure comprising a flat substrate in which a wave guide has been formed.
The invention also relates to a process for manufacturing such a device.
In numerous applications, polarizing optical elements are required. In recent techniques using optical fibres and/or integrated optics wave guides, it is further necessary to match these elements as well as possible to these structures and to their particular geometries and dimensions so that optical couplings may be optimized.
2. Description of the Prior Art
The devices used for polarizing light are divided into two main families: devices with selective absorption depending on the polarization, and devices with spatial polarization separation.
Devices of the first type have been more especially based on optical fibres. By way of example, among others, such a device has been described in the article by HOSAKA et al/ "Single-mode Fiber-Type Polarizer" appearing in the publication "IEEE Transactions on microwave theory and techniques", volume MTT30, no. 20 10, 1982.
Polarizers of this type comprise an optical fiber made from silica bared over a few centimeters, which, in this bared region, is subjected to polishing so that the air-silica interface is a flat diopter. A metal deposit is formed on this flat diopter.
The guided light wave may always be divided into a substantially transverse-magnetic polarization component and a substantially transverse-electric component.
The first component couples to a plasmon of the silica-metal interface, which plasmon is propagated while undergoing high attenuation. The first component is then attenuated in the same way. The second component, transverse-electric polarization component, on the other hand only undergoes low attenuation.
The attenuation rates obtained are typically of the order of 30 db. They depend naturally on the quality of the machining of the optical fiber and on the exact position of the interface with respect to the core region of this fiber.
The losses undergone by the second component are small on the other hand, typically of the order of 1 db cm.sup.-1.
Devices of the second family, i.e. devices with spatial polarization separation, have been formed either with an optical fiber structure base, or with a base of an optical wave guide structure integrated on a substrate. In these two variants, a birefringent crystal is disposed on a flat diopter adjacent either the core for an optical fiber or the integrated wave guide.
When it is a question of an optical fiber, this latter is first of all subjected to polishing, as was mentioned above, so as to create the flat diopter.
The birefringent crystal is chosen so that a first component of a given polarization light is propagated in a medium with a refraction index greater than the effective index of its guided propagation and so that another component, with orthogonal polarization, is propagated with a lower index.
Thus, the first component is no longer guided in the interaction zone with the birefringent crystal and is refracted therein. The second component, on the other hand, remains guided and passes through the interaction region without loss of energy other than the current transmission losses due to the guide or to the core of the fiber. These losses are in general negligible.
Attenuation rates of the order of 60 db or more may be obtained.
Polarizers of this type are described by way of examples, among others, in the article by BERGH et al: "Single-mode fiber-optic polarizer" appearing in the review "Optics Letters", vol. 5, no. 11, page 479, 1980, in so far as an optical fiber polarizer is concerned and in the article by UEHARA et al: "Optics Waveguiding polarizer" appearing in the review "Optics Letters", vol. 13, page 1753, 1974, in so far as the integrated optic devices are concerned.
The devices of the prior art which have just been described, although offering good performances, are however delicate to construct.
In so far as optical fibers are concerned, the machining for obtaining a flat diopter must be controlled with a very high accuracy during polishing. The position of the plane of the diopter must be determined with an accuracy on the order of a micrometer.
For optical fiber devices and integrated optics devices, it is difficult to obtain an appropriate crystal for all applications.
In fact, the conditions which the refraction indices must satisfy do not always correspond to existing crytals, at least crystals easy to machine and use.
Finally, in so far as the light wave guides are constructed in the form of an integrated optical structure, they are generally obtained by processes of the type in which a material, for example titanium, is thermally diffused in a surface region of a flat substrate, for example of lithium niobate.
This procedure may cause a modification of the surface state of this substrate and, correlatively, it becomes more difficult to obtain good optical coupling with the material of the crystal disposed on the surface of the substrate and interacting with the light guide region. It is then very often necessary to use index liquids inserted between the birefringent crystal and the surface of the substrate. It is also difficult to select liquids having the appropriate refraction index.
It should also be noted that the birefringent crystal is an external element, that is to say added and not integrated in the substrate as is the wave guide, which may be considered as an additional disadvantage.